Spin-flop transition in uniaxial antiferromagnets: magnetic phases, reorientation effects, multidomain states

摘要

The classical spin-flop is the field-driven first-order reorientationtransition in easy-axis antiferromagnets. A comprehensive phenomenologicaltheory of easy-axis antiferromagnets displaying spin-flops is developed. It isshown how the hierarchy of magnetic coupling strengths in theseantiferromagnets causes a strongly pronounced two-scale character in theirmagnetic phase structure. In contrast to the major part of the magnetic phasediagram, these antiferromagnets near the spin-flop region are described by aneffective model akin to uniaxial ferromagnets. For a consistent theoreticaldescription both higher-order anisotropy contributions and dipolar stray-fieldshave to be taken into account near the spin-flop. In particular,thermodynamically stable multidomain states exist in the spin-flop region,owing to the phase coexistence at this first-order transition. For this region,equilibrium spin-configurations and parameters of the multidomain states arederived as functions of the external magnetic field. The components of themagnetic susceptibility tensor are calculated for homogeneous and multidomainstates in the vicinity of the spin-flop. The remarkable anomalies in thesemeasurable quantities provide an efficient method to investigate magneticstates and to determine materials parameters in bulk and confinedantiferromagnets, as well as in nanoscale synthetic antiferromagnets. Themethod is demonstrated for experimental data on the magnetic properties nearthe spin-flop region in the orthorhombic layered antiferromagnet(C_2H_5NH_3)_2CuCl_4.